a) Field of the Invention
The present invention relates to an objective optical system which has a vari-focal function.
b) Description of the Prior Art
In recent years, there have been demands for endoscopes which permit observation not only of usual images but also of magnified images for precise diagnoses. Furthermore, there have been demands for endoscopes which have non-flexible tips which are short in length and have a small diameter. Accordingly, there have been demands for optical systems for observation of magnified images which have shorter total lengths and smaller outside diameters.
Also in the fields of video cameras and digital cameras, there are demands for lens systems which have shorter total lengths and smaller diameters.
As conventional examples of optical system for observation of magnified images, there are known optical systems disclosed by Japanese Patents Kokoku Publication No. Sho 61-44283, Kokai Publication No. Hei 4-218012 and Kokai Publication No. Hei 6-317744. Each of these conventional examples is a lens system which consists, in order from the object side, of a first lens unit having a positive refractive power, a second lens unit having a negative refractive power and a third lens unit having a positive refractive power, and is configured to perform a vari-focal function by moving the second lens unit along an optical axis.
Since the second lens unit is moved for a long distance for changing a magnification and a diverging light bundle is incident from the second lens unit having the negative refractive power onto a portion of the third lens unit which is remarkably higher than portions of incidence of the other lens units, each of these conventional examples has a defect that it has a large total length and another defect that the third lens unit has a large outside diameter.